Using Polymorphism with incompatible signatures - vb.net

Question

Say I have a class structure like this:
Public class Person1
inherits Person
public function Greeting(ByVal strGreeting As String)
return strGreeting
end function
end class
Public class Person2
inherits Person
public function Greeting()
return "Hello"
end function
end class
I want to be able to use Polymorphism to call: Greeting i.e.
dim P1 As Person = New Person1
dim P2 As Person = New Person2
msgbox(P1.Greeting)
msgbox(P2.Greeting)
However, Person2 accepts an argument, so the signatures are not the same. What is the solution to this?

Answer 1

Your person objects are instances of Types which just happen to inherit from some the same type. As an instance of Person1, the P1 object is not going to have any knowledge of methods contained in Person2 because it is a different Type.
I am not quite sure what you are after, but here is one way to construct it:
Public MustInherit Class Person
Public Property Name As String
Public Sub New(n As String)
Name = n
End Sub
' "I dont care HOW they do it, but child types
' must provide a Greeting() function"
MustOverride Function Greeting() As String
' overload: takes an arg/different sig
Public Overridable Function Greeting(strGreeting As String) As String
Return String.Format("Greetings, {0} {1}", strGreeting, Name)
End Function
End Class
Public Class Person1
Inherits Person
Public Sub New(n As String)
MyBase.New(n)
End Sub
' override with a specific form of address
Public Overrides Function Greeting() As String
Return "Hello, Mr " & Name
End Function
End Class
Public Class Person2
Inherits Person
Public Sub New(n As String)
MyBase.New(n)
End Sub
Public Overrides Function Greeting() As String
Return "Hello, Senorita " & Name
End Function
End Class
Then:
Dim Pa1 As New Person1("Ziggy")
Dim Pa2 As New Person2("Zoey")
Console.WriteLine(Pa1.Greeting)
Console.WriteLine(Pa2.Greeting("Ms")) ' uses base class overload
Console.WriteLine(Pa2.Greeting())
Output:
Hello, Mr Ziggy ' from P1
Greetings, Ms Zoey ' from base class
Hello, Senorita Zoey ' from P2
The base class more commonly might define a default Greeting for all the inherited types, then some base classes might override it and others not depending on the class needed.